Epigenetics - beyond genes
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Epigenetics - beyond genes
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Recent developments in epigenetics suggest that you may inherit more
than genes from your parents.
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Contents
Key text
Box 1: RNA interference and epigenetics
Box 2: RNA interference and plant technology
Box 3: The Human Epigenome Project
Activities
Further reading
Useful sites
Glossary
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Key text
Inside every plant and animal cell the genes provide instructions on
how to grow, multiply and function. But not all genes are used at all
stages of development, in all types of cells. Epigenetic factors can
regulate the amount of gene activity, influencing the growth and
appearance of an organism. What's more, epigenetic factors appear to be
inherited by the following generations.
Understanding epigenetics is fundamental to understanding how cells
work because malfunctions in epigenetic control of gene activity have
been implicated in cancer, cardiovascular disease and several inherited
genetic conditions.
Types of epigenetic factors
There are several epigenetic ways in which gene activity can be
prevented or controlled, including modification of histone proteins,
DNA methylation and RNA interference (Box 1: RNA interference and
epigenetics). For any of these methods of gene regulation, the absence
of the protein product of the gene causes a change in the function or
development of the cell.
Role of DNA methylation in regulating gene activity
DNA methylation prevents the expression of genes by altering the amount
of messenger RNA. Enzymes attach chemical tags called methyl groups to
the bases from which DNA is made. But not all bases in DNA are
methylated. The most common site for methylation to occur is a cytosine
base followed immediately by a guanine base - a combination of base
pairs known as a CpG.
The CpG combination of base pairs is relatively rare in most of the
human genome, but occurs with unusual frequency at points known as 'CpG
islands', which are often found in the promoter region of genes.
Promoter regions are found at one end of a gene and control the level
of gene activity.
The tagging of CpG's in promoter regions with methyl groups decreases
the amount of RNA made from the gene, so it is said to 'silence' the
gene. In normal cells, promoter regions are mostly free of methylation,
while CpG's outside the promoter region are almost always methylated.
DNA methylation in plants
DNA methylation in plants is more diverse than in animals. In addition
to methylating CpGs, plants also methylate the cytosine at CpNpG and
CpNpNp sequences, where N can be any base. Plants also have a greater
variety of enzymes involved in methylating DNA than animals.
Methylation of plant DNA occurs in transposon sequences, regions of
repeated DNA sequences and in the coding region of genes.
DNA methylation patterns are heritable
Once a gene has been methylated, all the daughter cells from that cell
retain the methylation, making it a heritable change. Changes made to
DNA are perpetuated every time the cell divides: eventually, many cells
carrying the modification will exist. Age and environmental factors can
change the amount of DNA methylation that occurs during a lifetime.
Inappropriate methylation of genes is implicated in diseases such as
cancer and atherosclerosis (hardening of the arteries). Some genetic
conditions are caused by inappropriate over or under methylation of the
same region of DNA, such as Prader-Willi and Angelman syndromes.
Related site: Prader-Willi syndrome
Reviews the epidemiology, diagnosis and genetics of Prader-Willi
syndrome.
(American Family Physician, USA)
Although most methylation is thought to be 'reset' when sperm and eggs
are formed by meiosis, there is evidence that the methylation pattern
of some genes can be inherited by offspring. This is causing a stir in
biology, because it suggests that environmental stresses such as
smoking or malnutrition experienced in a lifetime can have health
impacts on that person's descendants for several generations.
The link between DNA methylation and cancer
Cancer is now recognised as both a genetic and epigenetic disease.
While some types of cancers can be inherited, other cancers result from
changes to DNA that accumulate throughout life. Whether inherited or
spontaneous, cancer is caused by a change within a gene or series of
genes, resulting in uncontrolled cell growth and multiplication.
Only a small number of the roughly 20,000 to 25,000 genes in humans are
associated with cancer. There are three types of cancer-causing genes:
oncogenes, tumour suppressor genes and DNA repair genes. Increasing
evidence suggests that abnormal methylation of tumour suppressor genes,
which causes a loss of normal function, plays a pivotal role in the
development of many cancers. Related site: The Cancer Genome Project
Describes the project to detect mutations in the human genome that
cause cancer.
(The Wellcome Trust Sanger Institute, UK)
The DNA in cancer cells often has a methylation pattern radically
different to that found in normal cells. The promoter regions of genes
in healthy cells are normally free of methylation, while the rest of
the genome is heavily methylated. The reverse is true in most cancer
cells, where the promoter regions are heavily methylated and entire
regions of the genome can be abnormally suppressed or inactive.
DNA methylation, diet and the environment
Because DNA methylation can be affected by diet, stress and other
environmental factors - including heavy metals, pesticides, diesel
exhaust and tobacco smoke - it is one mechanism to explain how many
dietary and environmental risk factors contribute to the development of
cancer.
To maintain normal DNA methylation patterns, several essential
nutrients are required from the diet, including a source of methyl
groups (eg, methionine or choline) and folate. Folate - found in
green vegetables, legumes, oranges, and fortified juice and cereals -
has attracted attention because a diet low in folate is thought to
increase the risk of developing colorectal cancer.
Phytochemicals are also being studied in mice and laboratory-grown
cancer cells for their affect on DNA methylation. Genistein, one of the
main phytochemicals in soy products, reactivates genes silenced by
methylation and slows the growth of cancer cells. This is one mechanism
proposed to explain why death rates from prostate cancer are low in men
from countries with soy-rich diets, such as Japan.
Age related cancers
DNA methylation is a dynamic process, with the enzymes involved
constantly working to methylate and demethylate CpG sites throughout
the genome. These processes aren't perfect, and over time mistakes in
DNA methylation can start to accumulate. Inappropriate methylation
patterns can lead to inactivation of genes that should be expressed,
which poses a particular problem when those genes are tumour suppressor
genes vital for controlling normal cell growth. Age-related methylation
is now thought to be one of the reasons cancer risk increases with the
passing of the years.
The Human Epigenome Project
Research into epigenetics has already provided new and exciting
advances in plant technology (Box 2: RNA interference and plant
technology), potential cancer treatments and new tools for researchers
trying to identify the function of genes. In recognition of the
importance of DNA methylation in epigenetics, it is now the subject of
the multi-million dollar Human Epigenome Project (Box 3: The Human
Epigenome Project).
Related Nova topics:
Biology meets industry - genomics, proteomics, phenomics
More food, cleaner food - gene technology and plants
The Human Genome Project - discovering the human blueprint
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Box 1: RNA interference and epigenetics
RNA interference is a natural defence mechanism to control levels of
gene activity where small segments of RNA cause messenger RNA to be
degraded before it is translated into protein.
RNA as a regulator of gene activity
The discovery of RNA interference began in plants, when researchers
tried to engineer petunia plants to produce more coloured pigment in
flowers, by adding extra copies of a gene responsible for pigment
formation. Instead of an increased amount of pigment, they found that
many of the flowers lacked pigment. The extra copies of the gene were
somehow suppressing the activity of the original gene. Later,
researchers discovered that small RNAs were present in suppressed
plants, that were absent in non-silenced plants.
Continued research revealed an entirely new way of regulating gene
activity that involved small RNAs.
Types of small RNAs
Small segments of RNA generally come in two varieties: small
interfering RNAs from invading viruses and microRNAs that are encoded
by genes in the cell. As research continues, new types of small
interfering RNAs are being identified, particularly in plants.
Small interfering RNAs
Unlike DNA, RNA normally exists only in the single-stranded form in a
cell. Because the genetic material of some viruses is made from
double-stranded RNA, cells treat the presence of double stranded RNA as
a sign of infection by a virus.
When double-stranded RNA is found in the cell, it is chopped into short
sequences of between 21 to 24 bases in length, and is then used as a
guide to find and destroy single-stranded messenger RNA with the same
sequence of bases. This mechanism helps prevent the invading virus from
using the cell's machinery to reproduce.
Researchers can 'trick' the cell into destroying the messenger RNA
from one of its own genes by deliberately introducing double-stranded
RNA with the same nucleotide sequence as a gene to be silenced. Because
the messenger RNA is destroyed before it can be translated into a
protein, the normal appearance or function of the cell is changed.
Small interfering RNAs and methylation of DNA in plants
In plants, small interfering RNAs can also trigger the methylation of
DNA. Small RNAs that are similar in sequence to a region of DNA, cause
the DNA to be methylated. The details of the mechanism are not yet
fully understood, but it represents yet another way to control the
level of gene activity.
MicroRNAs
Thousands of microRNAs have been identified in many species, including
humans, but their role in the cell was a mystery until recently.
MicroRNAs are regulators of embryo development, cell replication, plant
development and stress responses. Because they are involved in timing
of cell development and metabolism, any change to them can trigger
cancer. Researchers have discovered that cancer cells contain less
microRNA than healthy cells, and each type of cancer has its own
distinctive microRNA fingerprint. MicroRNAs are also known to regulate
the expression of c-Myc, an oncogene that is implicated in 15 per cent
of human cancers.
The discovery of microRNA and its role in development has lead to a new
appreciation of parts of the genome that were once considered 'junk
DNA'.
Related sites
Flash animation: Mechanism of RNA interference (Nature Reviews
Genetics, UK)
RNA interference and gene silencing: History and overview (Ambion, USA)
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Box 2: RNA interference and plant technology
Hairpin RNA interference (RNAi) is a gene silencing technology
developed by CSIRO that is used to develop new plant traits. It makes
use of the ability of RNA to fold back on itself to form a
'hairpin' shaped piece of double stranded RNA. The hairpin RNAi
triggers the cell's RNA interference mechanisms to degrade messenger
RNA from the target gene, reducing or completely silencing its
expression.
Hairpin RNAi has been used to create plants such as the blue rose,
virus-resistant wheat and barley, and cottonseed that makes healthy
food oils.
Hey true blue
While roses have long been available in a wide range of colours, the
holy grail of rose breeders, the blue rose, has proved impossible to
achieve using standard plant breeding methods. Victoria-based company
Florigene Pty Ltd have used hairpin RNAi to create the world's first
true blue rose. These roses have their natural pigment removed by
hairpin RNAi, and genes from the pansy and the iris added to allow them
to make a blue pigment instead.
Virus resistant wheat and barley
Hairpin RNAi has been used to develop varieties of wheat and barley
that are immune to yellow dwarf virus. By targeting segments of viral
genes with RNAi, the transgenic cereal plants are made immune to this
costly disease.
Healthier cottonseed oil
Cottonseed oil is used to make cooking oils and margarines, but the
process used to treat the oil to prolong shelf life and make it
suitable for cooking creates unhealthy trans fats, which raise blood
cholesterol levels. Using harpin RNAi, CSIRO has developed a cottonseed
oil that is high in oleic acid, which makes it suitable for cooking
purposes without the need for treatment.
Genetic research
RNA interference is providing scientists with new tools to study the
function of genes. By observing the effect of 'knocking down'
(reducing) or 'knocking out' (eliminating) the amount of a protein,
researchers can deduce its normal function in the cell. RNA
interference offers advantages over the traditional method of inducing
mutations to silence genes, because it can be used to reduce, rather
than completely eliminate, gene function. This allows researchers to
study the function of gene products whose complete absence results in
the death or severe malfunction of the cell.
Related sites
CSIRO, Australia
About CSIRO's hairpin RNAi
Healthier cottonseed oil
Wheat with immunity to Barley Yellow Dwarf Virus
World's first blue rose
Omega-3 oils in grains
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Box 3: The Human Epigenome Project
Following the Human Genome Project, which was completed in 2003, the
latest addition to the big human biology research efforts is the Human
Epigenome Project (HEP), run by the Human Epigenome Consortium. The
project aims to identify, catalogue and interpret the DNA methylation
patterns of all human genes in all major tissues.
The Human Epigenome Consortium began with a pilot study in 2003 into
the methylation patterns of a region of chromosome 6. This region
carries genes crucial to the human immune system that have been
implicated in many diseases, particularly autoimmune diseases such as
diabetes and rheumatoid arthritis.
The study examined about 0.4 per cent of the genes in the human genome,
identifying CpG positions that are variably methylated and involved in
modifying gene activity. The study also developed automated methods to
rapidly and accurately identify methylation patterns in a genome,
simplifying the analysis of larger sections of the human epigenome.
In June 2006, the project released DNA methylation profiles of
chromosomes 6, 20 and 22 for 12 different tissue types. The data
accumulated by the project is publicly available for use in
non-commercial research efforts.
Related sites
Human Epigenome Project (Human Epigenome Consortium)
Human Epigenome Project - up and running (Public Library of Science,
Biology)
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Activities
Biological Sciences Curriculum Study (USA)
Activity 1: DNA sequences - allows students to see how researchers
use DNA base sequences to detect normal variation and harmful
mutations. Other activities available on this page are: 'Do genes
determine our future?', 'The case of Nathanial Wu' and 'Public policy:
genetics and alcoholism'.
The Human Genome Project: Biology, computers and privacy - this
comprehensive module provides background material for the following
activities: 'Genetic registries', 'Explaining the outliers', 'Genetic
anticipation', 'Who should control information?' and 'Making public
policy'.
National Human Genome Research Institute (USA)
The Human Genome Project: Exploring our molecular selves - a
multimedia education kit. You can download component parts separately
(eg, 'How to sequence a genome', 'Ethical, legal and social
implications'). (Requires QuickTime)
Oak Ridge National Laboratory (USA)
Your genes, your choices - presents seven case studies about genetic
information.
National Human Genome Research Institute (USA)
The ethical, legal and social implications of genetic knowledge -
using a number of vignettes, students learn about using logical
guidelines to evaluate information and arguments used in
decision-making about genetic information.
Genetic timeline - students construct a timeline that allows them to
see how technology, and specifically biotechnology, affects the
progress of scientific discoveries.
DNA Interactive, Cold Spring Harbour Laboratory, USA
DNAi teacher guide - provides activities about DNA structure, model
organisms, genome mining, human origins and dealing with controversy.
Public Broadcasting Service, USA
Explore a stretch of code - students locate specific base sequences
in a gene to learn about their function.
Camp-in curriculum: Genetics - provides activities about heredity,
observing differences and similarities in a population and how
inheritance works.
Genetic research: Decisions to be made - students learn about current
genetic research and its ethical implications.
The Human Genome Project: Talk show time - suggests different points
of view that students could present during a role play activity.
Nobel.org, Sweden
Control of the cell cycle - students take on the job of cell division
supervisor.
The genetic code - students learn about the DNA code by playing the
'Crack the code' game.
Science NetLinks, American Association for the Advancement of Science
Mitosis - students make physical representations of the chromosomes
in the stages of cell division.
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Further reading
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Australasian Science
August 2004, pages 23-24
Pre-natal origins of adult disease (by Marelyn Wintour)
Explains how adult health may be programmed by the environment in the
womb.
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Environmental Health Perspectives
March 2006
Epigenetics: The science of change
Provides a technical review of research on epigenetics, including
diagrams.
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New Scientist
24 May 2006
Safety scare over 'the new gene therapy' (by Peter Aldhous)
Reports on an experiment using RNAi that caused liver damage in mice.
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1 April 2006, page 17
Single RNA jab adjusts blood cholesterol (by Andy Coghlan)
Describes how an injection of RNAi molecules can block the gene that
makes 'bad' cholesterol.
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4 March 2006, page 15
Have we got cell division all wrong? (by Rowan Hooper)
Describes a new insight into the separation of chromosomes during
mitosis.
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6 January 2006, page 10
Men inherit hidden cost of dad's vices (by Rowan Hooper)
Describes how poor nutrition and smoking in early life may influence
the health of men's sons and grandsons.
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17 November 2005
The food you eat may change your genes for life (by Alison Motluk)
Suggests that swallowing a pill or eating a specific food supplement
may permanently change the expression of your genes.
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5 September 2005
Human stem cells become unstable in the lab (by Gaia Vince)
Looks at the effect of culturing stem cells for long periods in the lab
on genes known to cause cancer.
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2 August 2005
Famine increases the risk of schizophrenia (by Gaia Vince)
Reports on a study in China showing an increased risk of schizophrenia
for babies born during famine.
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11 June 2005, page 7
Toxic effects can pass down the generations (by Rowan Hooper)
Suggests that epigenetic changes are responsible for decreases in sperm
counts for at least four subsequent generations of male rats exposed to
pesticides.
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8 June 2005
New suspect implicated in the development of cancer (by Andy Coghlan)
Reports on three studies suggesting that microRNA misregulation can
cause cancer.
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31 May 2005
Embryonic stem cells pass key safety test (by Shaoni Bhattacharya)
Suggests that the methylation patterns of six genes in four embryonic
stem cell lines does not change when grown in the lab.
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11 April 2005
Pregnant smokers increases grandkids' asthma risk (by Gaia Vince)
Suggests that the effects of smoking when pregnant can be passed on to
children and grandchildren.
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27 November 2004, page 36-39
Unlocking the secret power of RNA (by Philip Cohen)
Reports on the growing awareness of a more important role for RNA in
the cell.
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10 November 2004
Unlikely ally rescues gene-blocking therapy (by Philip Cohen)
Suggests that cholesterol can be used to enhance the effect of
injecting RNAi molecules to treat diseases in humans.
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30 October 2004, page 47
Life sentence (by Alison Motluk)
Reports on diet during pregnancy and the increased risk of heart
disease and diabetes in children.
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15 September 2004
Gene technique to fight human blindness (by Peter Farley)
Reports on the first human trial of RNAi to treat a condition that
causes blindness.
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7 October 2003
Human gene on/off switches to be mapped (by Shaoni Bhattacharya)
Reports on the launch of the Human Epigenome Project.
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9 August 2003, pages 14-15
You are what your mother ate (by Philip Cohen)
Suggests that what mothers eat during pregnancy could have a lifelong
effect on the genes of their children.
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23 June 2003
Gene to halt ovarian cancer found (by Shaoni Bhattacharya)
Reports that a tumour suppressor gene is switched off by methylation in
nearly all human ovarian cancers.
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31 October 2002
Grandad's diet affects descendants' health (by Gaia Vince)
Suggests that the amount of food a boy eats in the years before puberty
influences his grandchildren's risk of diabetes.
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Scientific American
5 July 2005
Identical twins exhibit differences in gene expression (by Sarah
Graham)
Suggests that the differences observed between identical twins may be
due to different DNA methylation patterns.
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1 October 2004, pages 30-37
The hidden genetic program of complex organisms (by John Mattick)
Describes the regulation of genes by RNA encoded in 'junk DNA' and
its role in development and evolution.
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1 October 2004, pages 68-71
Hitting the genetic off switch (by Gary Stix)
Reports on companies considering the use of drugs and RNAi in therapies
to block the action of RNA.
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1 December 2003, pages 78-85
The unseen genome: Beyond DNA (by W. Wayt Gibbs)
Reviews the epigenetic control of gene expression by DNA imprinting and
methylation.
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1 November 2003, pages 26-33
The unseen genome: Gems among the junk (by W. Wayt Gibbs)
Reviews the role of RNA encoded in the 'junk DNA' and the role of
RNA in control of gene expression.
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August 2003, pages 26-33
Censors of the genome (by Nelson Lau and David Bartel)
Provides an overview of the RNAi mechanism in plant and animal cells.
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9 June 2003
Untangling the roots of cancer (by W. Wayt Gibbs)
Looks at the molecular causes of cancer.
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1 April 2003, page 14
Ma's eyes, not her ways (by Carol Ezzell)
Looks at the epigenetic differences between clones.
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15 July 2002
Killing the messenger (by Carol Ezzell)
Looks at the possible use of RNAi to treat cancer and HIV.
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Useful sites
Inheritance... more than just genes information sheet (CSIRO,
Australia)
Provides information about epigenetics, or non-gene factors, that
affect traits in plants.
http://www.csiro.au/csiro/content/file/pfbb,,.html
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Centre for Genetics Education, Australia
Genetics and cancer
Summarises how cancer develops and provides a list of important points
to remember about cancer.
http://www.genetics.com.au/factsheet/44.htm
X chromosome inactivation: 'Switching off' one member of the pair
of X chromosomes in females
Describes the process of inactivation of the genes on one of the X
chromosomes in females by methylation.
http://www.genetics.com.au/factsheet/09.htm
Genetic imprinting
Describes genetic imprinting caused by methylation of genes from either
a mother or father.
http://www.genetics.com.au/factsheet/14.htm
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Basic principles of genetics: An introduction to Mendelian genetics
(Palomar Community College, USA)
Provides information on genetics including the probability of
inheritance and exceptions to simple inheritance.
http://anthro.palomar.edu/mendel/Default.htm
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Rediscovering biology: Molecular to global perspectives (Learner.org,
USA)
Genetics of development
Examines how molecules work to coordinate the genetic information of an
entire organism.
http://www.learner.org/channel/courses/biology/textbook/gendev/index.html
Cell biology and cancer
Covers the causes of cancer and treatments, tumour biology and
angiogenesis.
http://www.learner.org/channel/courses/biology/textbook/cancer/index.html
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National Cancer Institute (USA)
A series of tutorials that use graphics and simple text to provide
information about cancer.
Cancer
http://www.cancer.gov/cancertopics/understandingcancer/cancer
Cancer Genome Project
http://www.cancer.gov/cancertopics/understandingcancer/CGAP
Cancer and the environment
http://www.cancer.gov/cancertopics/understandingcancer/environment
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Inside cancer: Multimedia guide to cancer biology (Cold Spring Harbour
Laboratory, USA)
Uses animations to describe aspects of cancer. Includes the sections
'Hallmarks of cancer', 'Causes and prevention', 'Diagnosis
and treatment and 'Pathways to cancer'.
http://www.insidecancer.org/
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Epigenetics: A web tour (Science magazine, USA)
A collection of articles and web resources on epigenetics, including
DNA methylation, RNA interference and histone modification.
http://www.sciencemag.org/feature/plus/sfg/resources/res_epigenetics.dtl
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Glossary
base (in DNA). Any one of four nitrogen-containing bases (adenine,
thymine, guanine and cytosine). The sequence of the bases in DNA
determines the sequence of amino acids in all proteins found in living
things.
base pairs. Two bases held together by weak chemical bonds. The double
helix shape of DNA is dependent on its two strands being held together
by the bonds between the base pairs. In DNA, the bases that pair are
adenine with thymine and guanine with cytosine.
chromosome. A long DNA molecule that contains the genes of the
organism. Chromosomes are visible in cells during cell division.
DNA (deoxyribonucleic acid). The nucleic acid forming the genetic
material of all organisms, with the exception of some viruses which
have RNA. DNA is present in the nucleus and other organelles such as
mitochondria and chloroplasts.
DNA repair genes. Encode proteins that correct mistakes in DNA caused
by incorrect copying during replication and environmental factors such
as by-products of metabolism, exposure to ultraviolet light or
mutagens. The DNA repair process must operate constantly to correct any
damage to the DNA as soon as it occurs. For more information about the
role of DNA repair genes in cancer see Genetics of cancer (Learner.org,
USA).
enzyme. A protein that acts as a catalyst. Every chemical reaction in
living organisms is facilitated by an enzyme.
epigenetics. Is the study of heritable changes in gene activity that
occur without a change in the sequence of the genetic material.
Epigenetics literally means 'in addition to genetics'.
gene. The basic unit of inheritance. A gene is a segment of DNA that
specifies the structure of a protein or an RNA molecule.
genetic conditions. Those conditions or diseases that result from
abnormalities in chromosomes or DNA, and are inherited.
genome. The total genetic material of an individual or species.
histones. Proteins found associated with DNA in eukaryotic cells that
play a role in gene regulation. The DNA winds around the histone
protein to form chromatin. For more information about the role of
histones see The nucleus (Kimball's Biology Pages, USA).
messenger RNA. RNA molecule that is transcribed from DNA and is used to
direct the synthesis of a protein.
meiosis. A division of the nucleus that involves the separation of
pairs of chromosomes into different cells. Meiosis takes place in the
reproductive organs of sexually reproducing organisms. Meiosis involves
two nuclear divisions, both of which may take place before division of
the cell itself is complete. The eventual result is four cells, each
with half the number of chromosomes present in the original cell.
Crossing over of chromosomes during meiosis creates new combinations of
genes in the progeny that were not present in either adult. For more
information see How cells divide: Mitosis versus meiosis (Public
Broadcasting Service, USA).
mutation. A change in the DNA sequence of a gene that may be harmful or
beneficial. It is the only process that actually leads to new forms of
a gene, and it is the ultimate source of all variation.
oncogenes. Mutated forms of genes which produce protein products that
normally enhance cell division or inhibit normal cell death. For more
information see Genetics of cancer (Learner.org, USA).
promoter. The DNA sequence adjacent to the coding sequence of a gene,
which interacts with inducers or repressors and RNA polymerase to
determine whether that gene is active or not.
protein. A large molecule composed of a linear sequence of amino acids.
This linear sequence is a protein's primary structure. Short sequences
within the protein molecule can interact to form regular folds (eg,
alpha helix and beta pleated sheet) called the secondary structure.
Further folding from interaction between sites in the secondary
structure forms the tertiary structure of the protein.
Proteins are essential to the structure and function of cells. They
account for more than 50 per cent of the dry weight of most cells, and
are involved in most cell processes. Examples of proteins include
enzymes, collagen in tendons and ligaments and some hormones. For more
information see Protein structure and diversity (Molecular Biology
Notebook, Rothamsted Research, UK).
RNA (ribonucleic acid). A nucleic acid similar to DNA. There are a
number of types of RNA, the major ones being messenger RNA, transfer
RNA and ribosomal RNA. RNA can serve as a messenger between DNA and
proteins, as a structural molecule, as an enzyme and as regulators of
gene expression. In some viruses RNA is the genetic material. For more
information see Introduction to RNA and its functions (University of
Newfoundland, Canada).
tumour suppressor gene. Genes that encode proteins that normally
repress cell division or enhance cell death. For more information see
Genetics of cancer (Learner.org, USA).
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Posted September 2006.
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